1. Field of the Invention
The present invention relates to a process for preparing polyhydroxycarboxylic acid which is valuable as a biodegradable polymer used for a medical material and a substitute for general purpose resins.
2. Related Art of the Invention
Polyhydroxycarboxylic acid has biodegradability in addition to excellent mechanical, physical and chemical properties and can be degraded under conditions encountered in the natural environment without adverse impact thereon and is finally decomposed to carbon dioxide and water by microorganisms. Consequently, the polymer has recently received attention in various fields of use such as a medical material and a substitute for general purpose resins in view of environmental protection, and is expected to greatly extend its future demand.
As a preparation process of polyhydroxycarboxylic acid, it has been known in the case of, for example, lactic acid and glycolic acid that the raw material acid is usually dimerized by dehydration to first give a cyclic dimer. Ring opening melt-polymerization of the cyclic dimer is then carried out in the presence of a catalyst to prepare a high molecular weight polymer. The process, however, is unfavorable in economy because great labor and expenses are required for the preparation of the cyclic dimer, lactide and glycolide. Further, some kinds of hydroxycarboxylic acid do not form a cyclic dimer and the process cannot be applied to such cases.
On the other hand, processes for preparing polyhydroxycarboxylic acid by direct dehydration of hydroxycarboxylic acid or its oligomer have been disclosed in Japanese Laid Open Patent SHO 59-096123 and 61-028521.
In these processes, however, the inherent viscosity of the resulting polymer is limited about 0.3 dl/g and thus the polymer is insufficient in mechanical strength and cannot be applied to some uses.
Further, the present inventors have applied for a patent (Japanese Patent Application HEI 4-337321) on a process for preparing polyhydroxycarboxylic acid having an average molecular weight of 30,000 or more by azeotropically dehydrating hydroxycarboxylic acid or its oligomer in an organic solvent in the presence of a catalyst in an inert atmosphere, treating the distilled solvent with a drying agent, and returning the treated solvent to the reaction system. The process, however, requires a reaction time of 17.about.50 hours in order to obtain polyhydroxycarboxylic acid having an average molecular weight of 120,000 and thus efficiency is not so good.
When a kettle type reactor equipped with a common stirring blade is used, the reaction is carried out under reduced pressure or under ventilation of an inert gas in order to remove low boiling components, i.e., water in the case of polyhydroxycarboxylic acid formation, which leads to a reduction in reaction rate. However, in the case of the kettle type reactor equipped with the common stirring blade, it is difficult to remove efficiently the low boiling components because of low efficiency of contact between the inert gas and the reaction mass and further because of the depth of the reaction mass even though pressure reduction is enhanced.